DE 100 41 007 C1 discloses a method for operating an anesthesia apparatus, in which a velocity, at which a piston of a piston-and-cylinder unit acting as a breathing gas feed unit is returned during an expiratory phase, is determined and preset for the most optimal recovery possible of exhaled breathing gas. Even though this method has proved to be excellent in practice, the method leads, as intended, to the above-mentioned optimal recovery of exhaled breathing gas and hence, due to the system, to a comparatively long gas change time.